Wireless or light communication systems, particularly infrared communications systems have a number of problems to deal with in effecting successful communications. One of the characteristics of wireless communications, particularly in infrared communications, is that the signal being received at a receiver may vary by many orders of magnitude because of differences in transmitter power, direction, atmospheric effects, distance, absorption, and other effects that interfere with signal transmission. In addition there is significant noise from other sources of heat and light that interfere with infrared communications. For this purpose, special circuitry has been used to enhance the reception of the true signal being transmitted. The signal being transmitted is typically in the form of digital pulses, which would, under ideal conditions, easily be perceived by the receiver. Because of the interference that may be present, as mentioned, conventional circuits use a comparator which compares the received signal to a threshold value.
In infrared communication, a photo detector is used to detect infrared signals received and produces electric pulses in response to the infrared signals. The electric pulses produced by the photo detector are typically amplified and the resultant amplified digital pulse stream is compared to an internal decision threshold voltage by means of a voltage comparator and based on the comparison made, the comparator will produced an output pulse if the threshold voltage is exceeded.
In other fields of communications a fixed threshold may be satisfactory. However, in the wireless or infrared communications fields, a fixed threshold can cause numerous problems due to the wide variations in the signal. These variations will cause dead zones, fading, and pulse widths that are too wide or too narrow to be handled by the receiving circuitry. When the signal strength is too high, because of a nearby transmitter, for instance, the pulse widths being received may be too large for the receiver circuitry to handle. Correspondingly, when the transmitter is either too weak because of positioning or distance, the pulse width determined by the comparator circuit would be too narrow for reliable processing.
U.S. Pat. No. 4,459,311 issued Oct. 17, 1995, assigned to Hewlett-Packard Company, Palo Alto, Calif., is directed to a fibre optic system of a fixed length that includes both a light source for producing optical signals, and optical fibre coupled to the light source to transmit the optical signals. A photo detector is coupled to the optical fibre to detect those optical signals and convert them to electrical signals. A delay line is coupled to the photo detector to delay the electrical signals from the photo detector for a period sufficient for the decision threshold circuit, which includes a peak detector, to detect the peak voltage of the first pulse received in a communication transmission. The threshold voltage is set to half of the peak voltage of the first digital pulse received. This threshold setting is maintained for the rest of the transmission. In addition, as the peak voltage is used a problem is encountered which requires compensation for a cumulative DC offset voltage.
As the reference is directed to a fibre optics system in which the system itself is a fixed configuration, i.e.: the transmitter, receiver, and optical fibre line are fixed to each other; the variations in signals expected would not result in a large change over short periods of time, e.g. during the currency of a transaction. The variations indicated by the reference vary by no more than 20 dB and this variation is not generally time related. It is apparently related to the length of fibre chosen for each system, and the strength of the transmitter. So a given receiver having an effective range of signal strength acceptance can be used in different fixed installations. The system disclosed apparently cannot compensate for variation in signals during a communication period as it sets its threshold only at the beginning of a communication period.
In the optical fibre communication system, once established the signal amplitude is constant and typically only one communication protocol is used such as SONET (SDH). However, in infrared communication, the communication distance may vary over time during the transmission. For instance in a mobile infrared telephone the users may be moving with respect to each other. The signal amplitude changes within a very large range over time and the receiver expected to operate in this environment must also handle a large number of different communication protocols. This in turn poses a problem which the reference does not appear to be able to handle. In the reference the question left unsolved is how long should the peak value detected for the first pulse be kept, and when should it be updated. As discussed, this is not a critical problem in a fibre optic communication system where one protocol is being used. However, in the infrared field, and when different communication protocols are being used, the pulse duration and pulse separation can be widely different. For the IrDA 2.4 Kbs protocol, the pulse duration varies between 1.41 .mu.s to 85.55 .mu.s. For the IrDA 1.152 Mb/s protocol standard the pulse duration may vary between 147.6 to 260.4 ns. This poses a serious problem for prior art systems such as the reference. Experimentation has shown that a peak detection system does not work properly in infrared communications.
It is therefore desirable to use the instant signal as a basis for adjusting the threshold. Another basic problem for wireless communications is due to the base line shift caused by the pAC coupling used to reduce environmental noise. Whereas, it is desirable to handle this situation as well in order to achieve successful communications flexibility peak detection circuits cannot compensate for this.